Messed up Blender models when exporting to Unity3D
This looks like you have flipped normals.

Blender draws double-sided by default, but you can enable Backface Culling in the Display panel (one of the panels right hand side of the view-port).

You can also enable drawing normals which draws a line in the direction each face points.

So you can use either of these draw options to check which way your normals are flipped.

This could be...


Normals flipped the wrong way in Blender, you can select all in editmode and run 'Mesh -> Faces -> Recalculate Normals' (CTRLN). OR if they are all the wrong way around: you can simply flip the normals. 'Mesh -> Faces -> Flip Normals')
Negative scale on the object. Blender detects negative scaled objects and shows the faces pointing the right way, but many game engines will show normals flipped if you have a negative scaled object.